Abstract: Disclosed is an agent for inhibiting decrease in measured values in immunoassays, which may reduce the influences by interfering substances in a test sample so as to promote the accuracy of the immunoassays, as well as an immunoassay and a reagent for immunoassays using the same, with which the decrease in the measured values due to the interfering substances is reduced. The agent for inhibiting decrease in measured values in immunoassays, caused by interfering substances, is an ionic surfactant having a molecular weight of 1000 to 100,000, the agent being a polymer in which a hydrophobic cyclic monomer(s) having an ionic functional group(s) is(are) polymerized.

Abstract: A measurement value lowering inhibitor for immunoassay method that is capable of enhancing the accuracy of immunoassay method through suppressing of any influences of interfering substances existing in an analyte sample; and using the inhibitor, an immunoassay method in which any measurement value lowering by interfering substances is suppressed and a reagent for immunoassay method. The measurement value lowering inhibitor for immunoassay method used to suppress any measurement value lowering by interfering substances is comprised of an ionic surfactant consisting of a polymer obtained by polymerization of a hydrophobic cyclic monomer having an ionic functional group, which has a molecular weight of 1000 to 10×104.